Mr. Perkins, the biology instructor at a posh suburban girl's junior
  college, said during class, "Miss Smythe, would you please name the
  organ of the human body which, under the appropriate conditions,
  expands to six times its normal size, and define the conditions."

  Miss Smythe gasped, then said freezingly, "Mr. Perkins, I don't think
  that is a proper question to ask me.  I assure you my parents will
  hear of this." With that she sat down red-faced.

  Unperturbed, Mr. Perkins called on Miss Johnson and asked the same
  question.

  Miss Johnson, with composure, replied, "The pupil of the eye, in dim
  light."

  "Correct," said Mr. Perkins. "And now, Miss Smythe, I have three
  things to say to you.  One, you have not studied your lesson.  Two,
  you have a dirty mind.  And three, you're going to be very disappointed
one
  day."